Physicians make use of catheters today in medical procedures to gain access into interior regions of the body to ablate targeted tissue areas. It is important for the physician to control carefully and precisely the emission of energy within the body used to ablate the tissue.
The need for careful and precise control over the catheter is especially critical during procedures that ablate tissue within the heart. These procedures, called electrophysiological therapy, are becoming more widespread for treating cardiac rhythm disturbances.
During these procedures, a physician steers a catheter through a main vein or artery (which is typically the femoral artery) into the interior region of the heart that is to be treated. The physician then further manipulates a steering mechanism to place the electrode carried on the distal tip of the catheter into direct contact with the tissue that is to be ablated. The physician directs radio frequency energy from the electrode tip through tissue to an indifferent electrode to ablate the tissue and form a lesion.
Cardiac ablation especially requires the ability to precisely monitor and control the emission of energy from the ablation electrode.
Conventional systems control radiofrequency power to the ablation electrode based upon root mean squared derivations of the rapidly fluctuating radiofrequency voltage and current. These root mean squared derivations represent real power (P.sub.Real) only if the radiofrequency voltage is in phase with the radiofrequency current. When a phase shift develops, the product of the root mean squared voltage and current (also called the apparent power, or P.sub.Apparent) differ from the real power (P.sub.Real) by a factor of the cosine of the phase angle (.phi.) between the radiofrequency voltage and radiofrequency current, or P.sub.Real =P.sub.Apparent *cosine .phi., or P.sub.Real =(V.sub.RMS *I.sub.RMS)*cos .phi.. Because, P.sub.Real takes in account the phase angle (.phi.), this Specification refers to P.sub.Real as "phase sensitive power." They cease having direct relevance to the control decisions that seek to maintain desired power levels at the ablation electrode. In fact, once large phase shifts develop, root mean squared derivations can lead to erroneous control decisions.